Problems of vertical configuration

Concentrations of mass

The problem in question is caused by high concentrations of the
total mass of the building on a given floor because heavy elements, such as
equipment, tanks, storerooms, files, etc., have been placed there. The problem
worsens the higher the heavy floor is, since the seismic response accelerations
also increase upward and there is consequently a greater seismic response force
at that point and a greater possibility of collapse.

The architectural design of these buildings should place whatever
areas involve unusual weights in basements or in separate structures near the
main body of the building. In cases in which for topographical reasons large
amounts of water must be stored at high elevations, independent towers should be
selected for this purpose instead of towers attached to the main building.

Weak columns

The seismic design of frames aims at ensuring that the damage
produced by strong earthquakes occurs to beams, not columns, since there is a
greater risk of building collapse from the latter type of damage. However, many
buildings designed according to seismic resistance codes have failed for this
reason. These failures can be grouped into two categories:

· Columns with less resistance than beams· Short
columns

There are several reasons why the value of free length is reduced
drastically and the result can be considered a short column:

· Partial lateral confinement of the column by dividing
walls, facade walls, retaining walls, etc.· Placement of slabs at
intermediate levels· Location of the building on a slope

Short columns are the cause of serious failures in buildings
subjected to seismic excitation, since their failure mechanism is fragile. The
most appropriate solutions in the case of all kinds of wall that impede the free
movement of the column consist basically of placing the wall in a different
plane from that of the column, or in separating the wall from the column by
means of joints. In the case of buildings with intermediate levels, the
architectural design should consider locating the columns outside the transition
line between the levels. Finally, on sloping land, the foundations of the
columns ought to be sunk at greater depths.

"Soft" stories

Several types of architectural and structural plans lead to the
formation of so-called "soft" stories, that is, stories that are more vulnerable
to seismic damage than others, since they have less stiffness, less resistance,
or both. The usual plans are:

· A story that is significantly taller than
others· Interruption of vertical structural elements on the
floor· Construction on a slope

The first case frequently arises because of a desire to place
greater masses at certain levels of the structure, usually for technical reasons
(equipment requirements, etc.) or aesthetic reasons (the building's appearance
at the access levels, etc.). As a result, stiffness on the floors in question
weakens due to the higher elevation of the vertical elements and of the
resistance.

The interruption of vertical elements of the structure has proven
to be the cause of multiple partial or total collapses in buildings subjected to
earthquakes. The reason is that the floor on which the elements are interrupted
has greater flexibility than the others, thus aggravating the problem of
stability; in addition and mainly, however, a sudden change in stiffness takes
place, causing a greater accumulation of energy on the weaker story. The most
common cases of such interruption, which usually occurs by virtue of size, form,
or aesthetic reasons, are the following:

Setbacks in the mass of a building are generally used because of
city planning requirements relating to illumination, proportion, etc. However,
from the standpoint of earthquakes, they cause sudden changes in stiffness and
mass and accordingly give rise to the problems mentioned above of concentrating
the destructive energy in the floors near the area of the sudden change. In
general, the aim should be to make the transitions as smoothly as possible in
order to avoid such concentration.

Inverted setbacks should be avoided in seismic areas, since they
also involve a serious risk of overturning, as mentioned with the distribution
of mass (see Figure 25).

Figure 25. Irregular structure

Graphic interpretation of "irregular structures or framing
systems," from the Commentary to the SEAOC Recommended Lateral Force
Requirements and Commentary. Reproduced in Arnold, Christopher and
Reitherman, Robert, Building Configuration and Seismic Design (New York:
John Wiley & Sons, 1982, p. 8). Reprinted with permission of John Wiley
&
Sons.